Electronic devices, including consumer electronic devices, appliances, and the like, often use permanent split capacitor (PSC) motors. For example, many evaporative coolers use PSC motors to provide air ventilation. The motor speeds of PSC motors are typically controlled by triacs. When the conduction angle of a triac changes, the output voltage of the triac changes, and the speed of the motor changes. The larger the conduction angle of the triac, the faster the motor runs. So the speed of the motor can be adjusted from low speed to high speed by changing the speed setting on a user interface panel of the evaporative controller.
A triac is turned on (i.e., conducts current) by applying a control signal to a gate terminal. The gate signal is generated by a circuit in the evaporative controller. The instance along each half cycle of the alternating current (AC) voltage signal at which the gate signal is applied to the gate terminal of the triac makes the triac conduct at that instance. Hence, the gate signal uses the zero-crossing of the AC voltage signal as the reference to determine the triac turn-on instance. The AC voltage to the motor can be changed by varying the delay angle of the triac on each half of the AC voltage signal in order to change the speed of the motor.
When the gate signal is applied to the gate terminal of the triac, the triac conducts and current flows through the triac to the load which is the PSC motor. When the load current (IT) flowing through the load reaches the triac's latching current (IL), IT is maintained even after the gate signal is removed. Once the triac is conducting and the gate signal is removed, as long as IT continues to flow and is higher than the holding current (IH) rating of the triac, the triac will continue to conduct. The triac will continue conducting until IT falls below IH, which occurs at the zero-crossing of the sinusoidal AC voltage signal. The diminishing AC voltage signal on the positive and negative cycles causes IT to gradually diminish and eventually fall below IH, and the triac turns off. When the triac turns off, there is no AC voltage applied to the load and no power consumed by the load, and therefore no heat is generated.
Triac-controlled PSC motors have long been used in evaporative coolers and other applications. However, as technology has advanced and as permanent magnet AC motors have developed, the limitations of using PSC motors have become more pronounced. In particular, permanent magnet AC motors are more efficient, quieter, and smaller, their speed can be controlled more accurately, and they have better mechanical properties than PSC motors. As demands for greater efficiency have increased, and as the cost of using permanent magnet AC motors has decreased, it has become desirable to replace PSC motors with permanent magnet AC motors. However, a permanent magnet AC motor cannot be driven by a triac. Specifically, the varying conduction angle of the triac cannot be used directly to control the speed of the permanent magnet AC motor. To achieve variable speed, a motor controller is required, including to perform converter and inverter operations, wherein the AC input is converted to DC, and the motor controller provides a sinusoidal voltage to drive the permanent magnet AC motor.
This background discussion is intended to provide information related to the present technology which is not necessarily prior art.